1. Field of the Invention
The present invention relates to the writing of intelligence information on the writing surface of a recording medium. More specifically, the invention is directed to a method and means for monitoring the focussing condition of the writing light beam impinging the writing surface of the medium.
2. Description of the Prior Art
Systems for monitoring the focus condition of a light beam used in reproducing prerecorded information from a videodisc by a videodisc player are well known, and a specific example can be found in U.S. patent application Ser. No. 298,405 filed Sept. 1, 1981 now U.S. Pat. No. 4,439,848 which is a continuation of U.S. application Ser. No. 131,513 filed Mar. 18, 1980, now abandoned, which, in turn, is a continuation of U.S. Ser. No. 890,670 filed Mar. 27, 1978, now abandoned, all filed in the name of Ceshkovsky et al. In the Ceshkovsky application, a collimated reading light beam from a laser light source is directed through a diverging lens employed to shape the beam to fully fill the entrance aperture of a microscopic objective lens which, in turn, converges the beam to a focussed spot on the surface of the videodisc. As the disc rotates, the pits or bumps along the track of the disc cause intensity modulation of the reflected beam which progresses along a portion of the incident light beam path until it reaches a polarizing cube which diverts the reflected beam out of the path of the incident beam and focusses it on a photodetector. As the intensity of the beam focussed on the photodetector varies, the intelligence information recorded on the videodisc is transformed into electrical impulses which through a series of electronic processes produces the video and audio portions of a recorded program.
In the player arrangement just described, the reflected light is split off from the incident light beam path in a region where the incident beam is diverging. In this manner, the reflected beam entering the polarizing cube, placed in the diverging portion of the incident beam, produces a converging light beam diverted to the photodetector. In analyzing the optics involved in such a system, the fact that a cone-shaped beam is produced by the diverging lens permits a pair of conjugate points to be developed, one at the surface of the videodisc and the other at the focus point on the photodetector. Thus, the light split off from the polarizing cube forms a focussed spot on the photodetector which has correspondence to the focussed spot on the disc. Of course, if the light beam is out of focus as it impinges the disc, the strength of the signal detected by the photodetector will be less. Accordingly, optimum focus and signal recovery can be gained by monitoring the signal level developed at the photodetector, adjusting the objective lens focussing the beam on the disc until a maximum signal is recovered, and then adjusting the spacing between the polarizing cube and the photodetector until the signal is again maximized. At the optimum operating point, the conjugate points of the optical path will be in focus at the surface of the disc and the photodetector simul- taneously.
In a player, the most common means of maintaining proper focus is to continually monitor the strength of the signal produced at the photodetector and, through a servo control loop, drive the objective lens along the axis of the reading beam until the recovered signal is maximized.
Another means of monitoring and maintaining focus in a videodisc player consists in passing the reflected beam of uniform divergence with circular profile through a cylindrical lens, thus increasing the divergence along the basal plane while leaving the divergence in the sagital plane unaltered. The beam is then directed through a passageway surrounded by four photosensors in quadrature, including a pair of vertically aligned sensors and a pair of horizontally aligned sensors. The beam thus illuminates the four sensors in spatial quadrature with the sensor arrangement positioned along the beam at a location where the beam is changing from basal focus to sagital focus to define a circular beam profile and cause equal outputs from the pairs of horizontal and vertical sensors. Depending on the position of the conjugate point at the disc surface then, the beam profile at the detector changes from horizontal elliptical profile to vertical elliptical profile. By electronically monitoring the difference signal between the horizontal and vertical sensor pairs, a focus error signal can be developed to return the objective lens to a position effective to cancel out any difference and return the beam to a circular profile.
All of the above-mentioned forms of maintaining focus of a light beam do so by operating in response to the condition of the reflected reading beam as it converges toward the photodetector. Furthermore, most of the known electro-optical focus monitoring and control systems employed in player apparatuses are placed in the long conjugate of the objective lens. This is made possible by the use of a beam splitting cube. The focus detectors and monitors of the prior art work effectively by analyzing changes in the divergence of the long conjugate of the objective lens thereby inferring the length of the short conjugate of the objective lens as dictated by well known laws of optics.
Another factor in the effectiveness of prior art focus detectors and monitors consists, in large part, in the strength of the signal recovered from the videodisc after impingement by the reading beam. The highly reflective metallized information-containing surface of the videodisc produces a high percentage of reflected light with which the photodetector and focussing apparatuses can reliably operate.
Since the present invention concerns the monitoring of the focus condition of a writing, or mastering, machine, there are several factors which would discourage the use of a polarizing cube at the same location along the beam path as in the disc players described above. First, the writing apparatus involves the use of a higher powered light source, 100 milliwatt argon ion laser tubes being quite common, as compared to the 1 milliwatt power capability of a reading beam of a player. Accordingly, inserting a polarizing cube and necessary quarter wave plate in the diverging portion of the writing beam, i.e., just before the objective lens, would introduce certain optical aberrations and distortions, such as the spherical aberration that can be expected after passing a light beam through any optical unit, aggravated by the large numerical aperture required in videodisc mastering, and would diminish the useful operating lifetime of the record laser due to the transfer efficiency of the polarizing cube.
The disadvantage of the spherical aberration introduced by placing a beam splitting cube in the divergent recording beam path has been recognized and overcome in the prior art by using objectives of infinitely long conjugate rather than the far more common 160 or 180 mm long conjugate. However, in such an arrangement, more elements are used, first to produce a wide collimated beam to illuminate the objective, and second to converge the diverted return beam to the focus monitor device in converging form.
Furthermore, since the disc surface upon which the writing beam is to impinge is not metallized but rather coated with a substantially clear photoresist compound, the disc surface is substantially transmissive, and making use of any possible reflected light energy is not readily apparent.
Finally, and although cost is not necessarily a large factor in the production of mastering machines, additional parts are necessary with the attendant complexities, and additional time must be invested in the assembly, alignment, and maintaining of the additional optical parts.
Because of the problems enumerated in the previous paragraph, it has been common practice in the prior art of recording laser discs to periodically determine optimum focussing position for the various optical elements affecting focus by producing a series of test discs and playing back each disc or portion thereof corresponding to a particular combination of settings for the various optical parameters, such as power and focus, involved. After locating a videodisc or portion thereof which has a high quality of reproducible signal therefrom, the log showing the various conditions and positions of the optics producing such high quality recorded program is referred to for returning the optics to that set of conditions. In this manner, although time consuming and not predictable, an acceptable focus condition is established. The empirically derived conditions necessary to produce acceptable focussing conditions, however, are extremely costly due to the labor time involved in setting up and determining the optimum focus conditions as well as in the loss of videodisc production due to down time of the mastering machine.
Accordingly, there is a need in the art to be able to quickly set the focus optics to an optimum condition on a repeated basis and to monitor the focus condition on a continuous basis.